Recently, an optical stereolithographic technique has attracted attention for fabricating a stereolithographic object based on three-dimensional CAD data by forming and stacking cured layers one by one by curing a photo-curable resin through ultraviolet laser scanning. Since the optical stereolithographic technique (hereinafter, the term “optical stereolithography” will also be referred to as “stereolithography”) enables easy and quick fabrication of prototypes without preparing a die or a mold, it is possible to reduce the time and cost required for a product development from the design to the production. Along with the rapid spread of three-dimensional CAD, the stereolithographic technique has been adopted in a wide variety of industrial fields such as automobile parts, electrical equipment, medical equipment, and so forth.
As the fields to the optical stereolithographic technique is applied are expanding, the performances demanded for the photo-curable resins are also increasing. Particularly, demanded are photo-curable resins having high curing rates and being capable of forming stereolithographic objects which are excellent in dimensional stability and dimensional precision when cured and tend not to be fractured even when an external stress such as bending is applied thereto, and which have excellent heat resistance and mechanical properties such as toughness and durability.
Moreover, from the viewpoints of environmental problems and safety, a technique which reduces the environmental burden and enhances the safety is demanded for the stereolithographic technique, for example, the process of producing a stereolithographic object and a photo-curable resin for use to form a stereolithographic object. For example, antimony compounds used as cationic polymerization initiators are generally toxic and exhibit toxic actions similar to those of arsenic and mercury, and have a great influence on the working environment in some cases. For this reason, there has been proposed a composition for stereolithography which contains no harmful components such as antimony compounds, so that the composition is excellent in safety and does not cause environmental contamination (for example, Patent Document 1).
In conventional compositions for stereolithography, an oil-soluble cationic polymerizable compound and an oil-soluble radical polymerizable compound are commonly adopted. Thus, the compositions for stereolithography exhibit such properties as being oil soluble. An organic solvent such as acetone or isopropyl alcohol needs to be used to wash a stereolithographic object produced by using such an oil-soluble composition for stereolithography and a stereolithography apparatus used for the production. The use of an organic solvent such as acetone or isopropyl alcohol is undesirable because it increases the burden to the environment, contaminates the working environment, and may adversely affect the health and safety of the workers. Hence, a water-soluble composition for stereolithography has been developed which contains a water-soluble radical polymerizable compound and an ionic surfactant (for example, Patent Document 2).
In addition, along with the progress of the stereolithographic technique, there are demands for photo-curable resins which are applicable to usages requiring higher heat resistance such as, for example, a stereolithographic object used as an engine part, and the like. For example, there have been proposed compositions containing a particular cationic polymerizable organic substance and a particular compound having two oxetanyl groups (for example, Patent Documents 3, 4, 5).